An inverter is an electrical device that may be used as a power converter in a device such as an appliance. Including elements such as switching elements, diodes, capacitors and control circuits, inverters may be used in various applications such as an appliance having a motor or an induction cooking apparatus. For example, in an appliance having a motor, the speed of the motor may be controlled by the inverter output. In an induction cooking apparatus, an inverter may be used to supply a high-frequency current to a coil in order to create induction heating for cooking.
There are numerous types of inverters including a half-bridge inverter, a full bridge inverter, and a polyphase inverter. In conventional inverter applications, the phase difference between the load current and the load voltage in the circuit is often needed in order to calculate output power.
A typical half-bridge inverter is illustrated in FIG. 1. As shown, power is supplied to a load Lload, Rload through an inverter 100. The inverter is generally configured to generate high frequency power from a DC or rectified AC power supply 110 at the required operating frequency of the load Lload, Rload. The load may be an induction coil, motor, or any other inductive load.
The inverter 100 is coupled to an AC source 110, for example a typical 120V power source, that supplies an AC signal to DC rectifier 120. The AC signal is transmitted to capacitor CDC. Capacitor CDC may act as a filter to prevent high frequency current from flowing towards the inverter and from entering the input. In addition, it provides a free-wheel path of high frequency current when an inductive load power is flowing back to the source side, for example when a motor is braking.
Inverter 100 may include switching devices Q1 and Q2, which provide power to the load Lload, Rload. The direction of the current flowing through the load Lload, Rload may be controlled by the switching of switching devices Q1, Q2. Gate driver circuits 130 and 140 provide switching timing of the switching devices Q1, Q2 based on a switching control signal provided from control circuit 150.
Switching devices Q1, Q2 may be insulated-gate bipolar transistors (IGBTs) or any other switching device. Snubber capacitors Cn1, Cn2 and capacitors Cr1, Cr2 are connected between a positive power terminal and a negative power terminal of switching devices Q1, Q2. Diodes D1, D2 are connected in parallel with switching devices Q1, Q2 and used as free-wheeling diodes. For example, diodes D1, D2 may eliminate a sudden voltage spike across the load Lload, Rload when the voltage is suddenly removed during a switching period.
A typical full-bridge inverter is illustrated in FIG. 2. A full-bridge inverter 200 may have many of the same elements as a half-bridge inverter and are identified with the same reference numbers. In addition to switching devices Q1 and Q2, a full-bridge inverter includes switching devices Q3 and Q4. Snubber capacitors Cn3, Cn4 may be connected between a positive power terminal and a negative power terminal of switching devices Q3, Q4. Diodes D3, D4 are also connected in parallel with switching devices Q3, Q4.
In typical applications, it may be important to monitor both the current and voltage load of the inverter system to determine phase difference between the current and voltage loads in order to provide feedback to adjust the power delivered to the load. In FIGS. 1 and 2, a current transformer CT may be used to measure the load current and the voltage across voltage divider 170 may be used in the load voltage measurement. The load current and voltage measurements are input into phase detector 160 to determine a phase difference. Phase detector 160 produces an output to the control circuit 150 proportional to the phase difference.
However, traditional phase angle measurements require both a load voltage value and a load current value. In addition, current transducers and voltage dividers may provide an inconsistent and inaccurate output due to nonlinear and phase shifting characteristics. Accordingly, output to the control circuit is inaccurate and causes inefficiencies within the inverter circuit.
Thus, a need exists for a system and method for phase angle detection that may be directly derived from a voltage detected across a shunt resistor. A system and method that could use a phase angle based on a directly detected voltage measurement to determine power delivery from an inverter would also be particularly useful.